The fusing of an electrical lead in a vehicle is usually implemented by means of a safety fuse which is typically designed for uniform loading of the electrical lead. Temporary pulse loads which are characterized by large current peaks are, however, particularly bad for the life span of a fuse because under certain circumstances these do not constitute a problem for the electrical lead, but can strain the fuse because the pulse loads come close to the actual triggering conditions such that more frequently recurring pulse loads reduce the life span of the fuse, and so the fuse ages under load peaks. If, for example, fuses melt within a range of 40-50 A, repeated current peaks in the range over 30 A lead to wear and ageing of the fuse. Consequently, inherent over-dimensioning occurs because the characteristics of the safety fuse and the lead are not matched to one another, and so the electric fuses are typically designed for larger currents and the dimensions of the electrical lead are also matched to this, and so greater consumption of materials and resources occurs.
One example of this poor matching between the safety fuse and the electrical lead is the protection of a motor with a nominal output of 140 W/12 A. Since the motor draws an initial current which exceeds the nominal current six-fold, it can not be protected with a 15 A fuse although the electrical lead could carry this pulse load. The consequence is that a larger fuse (for example in the range of 30 A to 40 A) and so an electrical lead with a larger cross-section has to be chosen (4 mm2).
A further aspect of this problem of matching the safety fuse and the electrical lead relates to thermal loads. While for short load peaks in the lead the flow of heat through the ohmic losses introduced is buffered by the large heat capacity of the electrical lead, the triggering range of the fuse is a “constriction” with a small heat capacity. Therefore, the fuse is greatly heated by the load peaks, and this can lead to ageing and faulty triggering.
For long-lasting, almost stationary overload situations with a low excess current, e.g. with 135% nominal current, the triggering range of the safety fuse can release its lost heat via the terminal lugs such that it takes several 1000 s before the fuse is triggered. The electrical lead can heat up, however, by 100° C. so that damage to the electrical lead can not be ruled out. It is therefore a technical object to provide intelligent protection for an electrical lead so that heating up of the electrical lead beyond a specific temperature can be avoided.
In the prior art circuits for this type of intelligent protection are known in which the load current is measured and averaged by means of smart MOSFETs (PROFETs). The temperature of the lead is then determined from an 12 t lost energy integration from the current profile by means of a software model. However, in comparison to normal MOSFETs, smart MOSFETs (Profets) are expensive components. Alternatively, standard MOSFETs can be used, the current of the electric conductor over the drop in voltage being measured by means of a shunt resistance or by means of the on-resistance (Rdson) of the MOSFET, but comparably expensive differential amplifiers are also required for this circuit.